Methods and devices of this kind are known for example from DE 199 44 733 A1, DE 198 14 594 A1 and DE 199 52 950 A1.
The recently tightened engine exhaust emission standards in particular have caused the motor vehicle industry to develop fuel injectors with fast and instantaneously operating final control elements or actuators. In terms of the practical implementation of such final control elements, piezoelectric elements (piezo actuators for short) have proved particularly advantageous. Piezoelectric elements of this kind are generally assembled as a stack of piezoceramic disks which are operated via a parallel electric circuit in order to be able to achieve the electric field strengths necessary for a sufficient displacement.
To control a capacitive load such as a piezo actuator which is used for actuating an injection valve, i.e. during charging and discharging of the capacitive load by means of a load current, considerable requirements are placed on the control electronics. An injection valve operated by a piezo actuator is used in internal combustion engines for injecting fuel (e.g. gasoline, diesel, etc.) into a combustion chamber. This places very exacting requirements on precise and reproducible opening and closing of the valve and therefore also on the control electronics, with voltages ranging up to several 100 V and, transiently, load currents for charging and discharging of more than 10 A having to be provided. Control mainly takes place in fractions of milliseconds. At the same time, the current and voltage must be fed to the actuator in as controlled a manner as possible during these charging and discharging processes.
DE 199 44 733 A1 discloses a circuit arrangement for controlling a piezo actuator in which the actuator is charged by a charging capacitor via a transformer. For this purpose a charging switch disposed on the primary side of the transformer is controlled by a pulse width modulated control signal. Both the charging switch and the discharging switch are implemented there as controllable semiconductor switches. Predefined energy packets are fed to and removed from the piezo actuator during charging and discharging respectively. The disclosed arrangement is based on the principle of a “bidirectional flyback converter” and allows precise apportioning of energy during charging and discharging of the piezo actuator.
DE 198 14 594 A1 describes a circuit arrangement for charging and discharging a single piezoelectric element. The control circuit disclosed is based on a half-bridge output stage which controls the piezo element via an inductor (choke), said choke being used primarily to limit the charging current occurring during charging and the discharging current occurring during discharging. Charging and discharging take place in a pulsed manner, i.e. by repeated opening and closing of a charging switch during charging and of a discharging switch during discharging, respectively. This again allows precise apportioning of energy during charging and discharging of the piezo actuator.
DE 199 52 950 A1 discloses a control unit for a piezo actuator in which the piezo actuator is controlled by an output stage designed as a “flyback converter”. The flyback converter with transformer used here enables the electrical energy supplied during the charging process to be largely recovered during discharging, temporarily stored in the converter and reused for the subsequent charging process. To charge the piezo actuator, a charging switch connected in series with the primary side of the transformer is operated intermittently. When the charging switch is closed, the current flowing on the primary side is compared with a reference current value. When the primary current attains the reference current value, the charging switch is opened again. This process is carried out repeatedly, so that each charging operation is effected by consecutive (secondary side) partial charging current pulses corresponding to consecutive partial charging power pulses. The time integral of each secondary-side partial charging power pulse represents an energy pulse on the secondary side of the transformer, the value of which is defined by the prevailing reference current value. In a first embodiment (FIG. 2,) the reference current value is set to a constant value during the charging process so that, on the secondary side, consecutive pulses of constant energy for charging the piezo actuator are generated. In another embodiment (FIG. 3), the charging process is started with a relatively large energy pulse which is followed by successively smaller energy pulses. In yet another embodiment (FIG. 4), an essentially cosine waveform of the reference current value is specified.
The problem with comparatively fast control of the capacitive load, as is required particularly for actuating an injection valve of an internal combustion engine, for example, is the risk of post-oscillation of the actuator because of mechanical and/or electrical resonances at the end of each charging or discharging process. When an actuator is controlled in such a way that a charging phase is followed by a holding phase and finally by a discharging phase, this may result in e.g. post-oscillation in the holding phase and/or discontinuities during activation or deactivation of the actuator. These effects are more pronounced the more rapidly the charging and discharging processes brought about by the pulsed current flow take place. In addition, many capacitive loads in practice possess a variable large signal capacitance or nonlinearities, which makes it difficult to achieve a particularly well defined energy input or output response and tends to increase and complicate the abovementioned oscillation or post-oscillation effects. When the capacitive load is a piezo stack, not only nonlinearities but also polarization losses and creep effects, etc. occur. Finally, for controlling an actuator for an injection valve of an internal combustion engine, transfer function effects of other coupling elements (e.g. hydraulic converters, hydraulic backlash compensation, levers, coupling rods, etc.) are also present.